The longevity of the engine really depends on the who made the car. I used to have a VW parts hauler which had the engine rebuilt at least 2.724 times!
The supposed ideal angle is (2*360)/number of cylinders. So for a V6 it would be 120 degrees. I don't know why the boxer would be so smooth but it is. Maybe it's one of those bumblebee things where we know that it should not be able to fly but does!
I too have heard the pistons boxing each other explanation for why it's called the boxer. I've always had a hard time visualizing this. Somehow my VW experiences have tarnished this image! The other advantage to the flat 6 is that you could drop it out of the body with nothing but a floorjack and two jackstands. This is a 20 minute job if you've practiced it a few times
Another advantage (on a VW, never tried it on a Porsche) is you could if forced to, install one just one new barrel and piston if you needed to. I guess it's possible in principle on a Porsche aircooled engine, but chances are slim that one would have an extra cylinder barrel and piston in the car!